Optical systems are presently being employed in the communication of voice and video information as well as in the high speed transmission of data. Optical communication systems are desired because of the wide bandwidth available for the information signal channels.
Although this wide bandwidth is available, many of the existing optical fiber systems use only a single channel per optical fiber. Typically, this channel is transmitted at a wavelength of 1310 nm in one direction from a transmitting end to a receiving end and requires a second optical fiber to achieve bi-directional communication; however, recent increase in telecommunications traffic has resulted in a need for further fiber resources. One way this need was met, was to install additional optical fiber cables. Another was to increase the number of channels carried by some fibers.
Recently, technologies that can add additional channels to existing optical fiber cables already in the ground, have gained acceptance. These technologies seek to provide more than one channel on a single existing optical fiber and are therefore aimed at enhancing the efficiency of the existing fiber optic cable network. These technologies include wavelength division multiplexing (WDM) and bi-directional transmission.
When a number of wavelengths are multiplexed and transmitted on a single optical fiber, customarily, these channels must later be demultiplexed into separate channels or wavelengths of light. For example, it may be cost effective to transmit signals of wavelength 11, 12, 13, 14, 15, and 16 (1 denoting a wavelength, lambda) along a single optical fiber, however, demultiplexing means are required to separate the light into six separate channels. Of course, it is desired to perform this demultiplexing at a minimum cost and with as little signal loss as possible.
Various types of optical filters have been contemplated and used to separate light of differing wavelengths. Unfortunately, coupling and other losses associated with many of these arrangements have led to devices that are less than satisfactory.
As of late, in-fiber Bragg gratings have become more prevalent in the field of fiber optics. An optical system utilizing Bragg gratings in combination with an optical circulator is shown in U.S. Pat. No. 5,283,686 issued Feb. 1, 1994 in the name of David Huber and assigned to General Instrument Corporation, Jerrold Communications, Hatboro, Pa.
It is an object of this invention to provide a multi-channel demultiplexor that is capable of demultiplexing a multi-channel signal with less loss than known prior art demultiplexors.
It is a further object of the invention to provide a multi-channel demultiplexor that is configured in a manner to minimize loss normally associated with serial-type architectures.
In accordance with one aspect of the invention, a multi-wavelength optical filtering device for optical signal transmission comprises an optical circulator having at least 4 ports, one of the at least 4 ports being an input port for receiving a plurality of wavelengths of light, another port being an output port, and at least two of the other ports each being coupled to a different wavelength selective element, preferably an in-fiber Bragg grating, each such element having different transmission characteristics.
In accordance with another aspect of the invention, the device further comprises a second optical circulator having at least three ports including an input port, and means for coupling one of said wavelength selective means of said device to said input port of said second optical circulator.
Preferably, each of the optical circulators has at least four ports. The number of the circulators coupled similarly as the first and second circulators can be significant. For the ease of understanding, the coupling of the circulators according to the invention, via the grating of one circulator being coupled to the input port of another circulator, will be termed "branching" or "parallel coupling" as opposed to the serial linking illustrated in FIG. 1.
The wavelength-selective means of the present invention (preferably in-fiber Bragg grating reflectors) are selected as necessitated by the specific arrangement to either reflect one particular wavelength and transmit all the other signals, or transmit one particular wavelength and reflect all the other signals.
The arrangements of the present invention can operate both in the multiplexor and demultiplexor mode.